brother pls download firmware files that suite your product code from
Nokia Firmwares
and try flash with infinity dont forget to erase all ( tick on all fileds root,emmc...etc). if its doesnt recover your imei try from old firmwares one by one. (50% ) it will recover your imei..
or flash n900 emmc first using linux or mac system or windows7...
there are sevral issues to flash emmc of n900 in windows that happened bcoz of driver issue.. its solved if you use linux or mac..
try to flash emmc with old firmwares it will restore the imei if you have luck. or if you know its factory made firmware version not new means bought handset on 2009 and in 2010 its updated to new... u have to flash it with 2009 firmware hope you got what i mean...
and if you guys have no problems with imei always flash it using latest firmwares. downgrades brick your phone.....
infinty is the best dongle for flash n900 and hence mxkey.......
if none of this works private message me... i will help you with my little knowledge....